Many people switch their air conditioning off in October and forget about it until June. But most modern split systems are reversible, and the Energy Saving Trust notes that air-to-air heat pumps are sometimes referred to as air conditioning. In heating mode, the same unit moves heat from the outside air into the room.
This guide explains how that works, how to estimate running costs honestly, what defrost cycles are, which settings help and where air-to-air heating fits alongside your existing heating.
How a heat pump heats
In heating mode the refrigerant cycle runs in reverse. The outdoor unit absorbs heat from outside air, even cold air, the compressor raises its temperature, and the indoor unit releases it into the room. Because the system moves heat rather than making it from scratch, it can deliver more heat energy than the electricity it uses.
Each model has a stated operating range for heating, and output falls as the outdoor temperature drops. Check the manufacturer’s data for the model you’re considering.
COP and SCOP in plain terms
The coefficient of performance (COP) is heat out divided by electricity in. A COP of 3 means 3 kWh of heat for every 1 kWh of electricity. A plain electric heater turns 1 kWh of electricity into 1 kWh of heat. COP changes with the weather, so manufacturers also publish a seasonal figure, SCOP, averaged over a heating season. Our guide to SCOP, SEER and EPCs explains the ratings.
Working out the running cost
You can estimate it with one sum: heat needed (kWh) ÷ COP × your electricity unit rate. For example, if a room needs 10 kWh of heat in a day and the system averages a COP of 3, it uses about 3.3 kWh of electricity. Multiply that by the unit rate on your bill. Compared with a gas boiler, the answer depends on both your gas and electricity prices and on how efficient the boiler really is, so run the sum with your own tariff. See also our running costs guide.
Defrost cycles are normal
In cold, damp weather, ice builds up on the outdoor coil. Every so often the system pauses heating to melt it. You may notice the indoor fan stop, a whoosh of refrigerant, and steam rising from the outdoor unit. That’s normal, and heating resumes after a few minutes. If the outdoor unit is encased in ice that doesn’t clear, call an engineer; see ice on an air conditioning unit.
Want it done properly? AirCon Doctor installs, repairs and services air conditioning and heat pumps across London and the South East, from our base in East Grinstead. We’re an F-Gas certified company and NICEIC approved. Get a fixed-price quote.
Settings that help
- Hold a steady temperature rather than switching on and off. Heat pumps work most efficiently running gently for long periods.
- Use auto fan and set the louvres to point down; warm air rises.
- Don’t turn it right up to warm up faster. It won’t, and it may overshoot.
- Use timers to start before you need the room.
- Keep filters clean. Dust cuts airflow and output in winter too.
Looking after the outdoor unit in winter
In heating mode the outdoor unit drips water from defrosting. Make sure it drains away from paths and steps, where it can turn to ice. Keep leaves and snow from blocking the coil and the base. Don’t chip ice off the fins. A unit raised on a stand or bracket drains better and is less likely to be buried in snow.
Where it fits with your existing heating
Air-to-air heating suits rooms you use a lot, extensions radiators struggle with, garden rooms and home offices. The Energy Saving Trust notes that air can’t carry as much heat as a wet central heating system, and most units don’t heat water, so many homes use it alongside their existing heating.
If you want to replace fossil-fuel heating entirely, the Boiler Upgrade Scheme offers £2,500 for an air-to-air heat pump, but only where it replaces the existing fossil-fuel heating, meets the home’s full heating demand and is fitted by an MCS-certified installer. It isn’t available as an add-on alongside a boiler. See our grant guide or air-to-air heat pumps.
A service before winter
Heating mode puts the system through different conditions from summer. A service checks the refrigerant circuit, coil, fans, drain and electrics, and makes sure defrost works. Our annual service costs £95–£150 + VAT.
Quick answers
Can air conditioning heat my home in winter?
Most split systems are reversible heat pumps and can heat rooms efficiently. Many homes use them alongside existing central heating.
Why does my unit stop heating and steam outside?
That’s a defrost cycle, melting ice from the outdoor coil. It’s normal, and heating resumes after a few minutes.
Is air conditioning heating cheaper than gas?
It depends on your tariffs, the system’s COP and your boiler’s efficiency. Divide the heat needed by the COP, then multiply by your electricity rate to compare.
Can I get a grant for air-to-air heating?
The £2,500 Boiler Upgrade Scheme grant applies only where the system replaces fossil-fuel heating, meets the full heating demand and is MCS installed.
Related services
More advice
- Heat Pump vs Air Conditioning: What’s the Difference?
- The £2,500 Boiler Upgrade Scheme Grant for Air-to-Air Heat Pumps
- Air Conditioning Energy Efficiency: SCOP, SEER and EPC Explained
- How Much Does It Cost to Run Air Conditioning in the UK? 2026 Guide
- Ice on Your Air Conditioning Unit: What It Means and What to Do
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.